The 4H-3,1-benzoxazine-4-ones and the various derivatives thereof are known in the art. These benzoxazines generally are prepared, for example, by reacting a mole of anthranilic acid with a substantial excess, i.e. two moles or more of an acylating compound, e.g. an aryloyl chloride in order to produce high yields of 2-aryl-4H-3,1-benzoxazine-4-ones. However, when an excessive amount of the acylating compound is used, i.e. one mole in excess of the stoichiometric amount needed to obtain the benzoxazines, they are contaminated with the corresponding carboxylic acid which is difficult to separate from the product.
Attempts to avoid this problem by decreasing the acylating compound to a stoichiometric amount of about one mole, resulted in yields of benzoxazine of only 30 to 40%. In theory, this low yield is believed to be due to an initial competitive reaction, i.e. the initial acylation of anthranilic acid resulting in a reaction mixture containing intermediates only one of which continues to react to form the benzoxazine while the other needs the addition of a second mole of acylating compound in order to form the benzoxazine in desired yields. In other words, in a 1:1 mole ratio reaction of the anthranilic acid with the acylating compound, less than about 1/2 of the acylating compound is converted to benzoxazine with the remaining being converted to carboxylic acid. To illustrate that only about 1/2 of the acylating compound is converted to benzoxazine, the anthranilic acid was reacted first with one mole of o-nitrobenzoyl chloride followed by reacting that mixture with a mole of benzoyl chloride. The only product obtained from that reaction was 2-(o-nitrophenyl)-4H-3,1-benzoxazine-4-one. This shows that when one mole of anthranilic acid is reacted with one mole of acylating compound only one of the intermediates initially formed is converted to the benzoxazine, i.e. nitrophenyl-substituted benzoxazine. Therefore, since it was recognized that the organic groups, e.g. aryl groups of these acylating compounds do not mix as illustrated by the nitrobenzoyl chloride reaction, it was prudent in preparing benzoxazines first to react the anthranilic acid with the more expensive acylating compound, e.g. the acyl chloride in a 1:1 ratio to insure that all of said acylating compound is converted and then to complete the reaction with the addition of a less expensive acylating compound such as acetic anhydride.